That is not true at all Ankur. Assuming he used YUMI like I do for bootable flashkeys, you can put a crap ton of other data on the key and still be able to boot it. I have just over 12GB of pics and txt files with Ubuntu and DBAN on my key and they both boot just fine.
Not only me, but quite many friends tried it and it didn't work after that, not to load an OS but to get past the admin password to of a system. Mostly we did the wrong way.
Most of the modern systems will allow other files on a boot flashkey. I know YUMI does, and I assume most others keep up with them. Really YUMI is the only USB boot program that makes sense to use.